This invention relates to semiconductor lasers and more particularly to a vertical cavity surface emitting laser (VCSEL).
A VCSEL is a semiconductor laser that comprises a multilayer semiconductive element that includes semiconductive layers that serve as the gain medium sandwiched between reflective mirrors, desirably distributed Bragg reflective layers, that form a cavity resonant at the optical wavelength desired for the laser for providing a standard wave, at a single fundamental mode, in a direction vertical to the layers. Such light can be readily coupled to an optical fiber for utilization. For efficiency, the mirrors should have reflectivity in excess of about 99.5 percent. Such high reflectivity mirrors are difficult to grow in the same epitaxial process used to grow the grain medium in long-wavelength lasers, for example at 1300 or 1500 nanometers, wavelengths important for use in optical fiber transmission systems. Accordingly, VCSELs designed for operation at long wavelengths typically have used for the mirrors either evaporated layers of dielectric material or of lattice mismatched semiconductors. Mirrors of this kind make difficult the electrical injection of charge carriers into the gain medium, the usual manner of creating the population inversion in the gain medium necessary to achieve the desired stimulated emission of radiation critical to laser operation.
To avoid this problem, there have been proposals to provide the necessary population inversion in the gain medium by optical pumping at a shorter wavelength. In such a VCSEL, there are provided a pair of cavities. The first is tuned to the desired long wavelength and includes a first medium suitable for being optically pumped at a shorter wavelength to provide light at the longer wavelength. The second cavity is tuned to the shorter wavelength and includes a medium for lasing at the shorter wavelength by the electrical injection of charge carriers, thereby providing light for optically pumping the first gain medium.
Long-wavelength optically pumped lasers of this kind have been described in U.S. Pat. No. 5,754,578 that issued on May 19, 1998. In a first arrangement described, in which the short wavelength cavity is superposed on the long wavelength cavity, the short wavelength radiation is emitted from the bottom surface of the short wavelength laser and transmitted through the top surface of the underlying long wavelength gain medium and the desired long wavelength radiation typically exits from the bottom surface of the long wavelength cavity.
In a first exemplary embodiment of this first arrangement, all the layers used to provide the mirrors in both cavities are made from the GaAs/AIGaAs system and the fabrication employs two wafer fusion steps.
In a second exemplary embodiment of this first arrangement, the bottom long wavelength mirror of the long wavelength VCSEL is fabricated from either the InP/InGaAsP system or the InP/InGa AIAs system and is grown in the same epitaxial step as the long-wavelength gain medium. The upper mirror, which is grown in the same epitaxial step as the short-wavelength VCSEL, is attached to the long-wavelength gain medium by wafer fusion. The remaining mirrors of the long-wavelength VCSEL and the short wavelength VCSEL are fabricated from the GaAs/AIGa As system.
In a third exemplary embodiment of the first arrangement, the long-wavelength mirror is attached to an upper GaAs/AIGaAs mirror by metal bonding; and the other long-wavelength mirror is either an epitaxially-grown InP/InGaAsP or InP/InGaAIAs mirror, a wafer-fused GaAs/AIGaAs mirror or a metal-bonded GaAs/AIGaAs mirror.
In a second arrangement, short wavelength light emitted from the top surface of an underlying short wavelength VCSEL is transmitted through the lower mirror of the long-wavelength VCSEL. The lower mirror of the long-wavelength VCSEL is grown in the same epitaxial step as the upper mirror of the short-wavelength gain medium VCSEL and is fabricated from the GaAs/AIGaAs system.
In an exemplary embodiment of the second arrangement, all the mirrors in the structure are from the GaAs/AIGaAs system, except for the top mirror of the long wavelength VCSEL. The top mirror of the long wavelength VCSEL is either (1) a wafer-fused GaAs/AIGaAs mirror, (2) an epitaxially-grown InP/InGaAsP or In P/InGaAIAs mirror, or (3) a sputtered or evaporated dielectric mirror. Any of these three mirrors can be supplemented with a metal reflector at the top of the stack to increase reflectivity.
It is characteristic of the various structures described in such prior art that the long-wavelength cavity includes an outer reflective boundary that is tuned to be highly reflective selectively at the long wavelength that is to be emitted as the output. There is no attempt made to establish in the cavity of the long wavelength cavity resonant conditions at the shorter wavelength so that a standing wave of the shorter wavelength is established in the cavity of the long-wavelength laser.
In these structures, each cavity is essentially independent of the other, and is designed to confine the electric field of the emitted light essentially entirely within its boundaries and to peak it within the gain medium included in its cavity. As a consequence this tends to limit the efficiency of the short wavelength light pumping process.
Additionally in the prior art structures, no attempt had been made to form all but the upper reflective mirrors as layers grown epitaxially on a monocrystaline substrate to make feasible a relatively thick gain medium in the long-wavelength VCSEL.
The present invention seeks to improve a long-wavelength VCSEL of the kind in which the long-wavelength VCSEL is optically pumped by a short-wavelength VCSEL, by improving the efficiency of the conversion of the short-wavelength pumping light to long-wavelength output light by changes in the optical structure that make possible efficient use of a thicker gain medium. Basically this is done by a cavity design that effectively couples together the cavities of the two VCSELs such that the electric field of the pumping light emitted by the short-wavelength VCSEL extends at significant strength beyond the cavity of the short-wavelength VCSEL and into the gain medium of the long-wavelength VCSEL.
This is done by stacking together on a common substrate the long-wavelength laser supported over the short-wavelength laser and designing the outer mirror of the long-wavelength laser cavity to cooperate with the inner mirror of the short-wavelength mirror to provide constructive buildup of the electric field of the short-wavelength laser in the long-wavelength laser cavity. To this end, the outer mirror of the long-wavelength laser cavity is a broad band mirror that is highly reflective of both the short- and the long-wavelength light involved in the device and is designed to establish a standing wave of the short-wavelength light in the cavity of the long-wavelength laser.
In an exemplary embodiment, a VCSEL designed to emit output light of a long wavelength, typically 1.3 xcexcm, comprises a single crystal substrate, on top of which are stacked a plurality of monocrystalline semiconductive layers, advantageously all grown by molecular beam epitaxy, topped by a dielectric mirror that forms an essentially monolithic structure free of fused layers. These multilayers form a first lower VCSEL, designed to use electrical pumping and to lase at the shorter wavelength and to optically pump the second upper laser that is to provide the longer-wavelength light that is to be the output.
The multilayers of the stack that form the first VCSEL typically comprise a first section that forms a mirror that is highly reflective of the shorter wavelength light, a second section that forms the cavity including the gain medium for such light, and a third section that forms a mirror that is both sufficiently reflective of the short-wavelength light to establish lasing in such gain medium and sufficiently transmissive to permit laser light of the short wavelength to penetrate into the second VCSEL. The first and third sections serve as reflective boundaries of a cavity resonant at the shorter wavelength, as in a conventional VCSEL.
The layers of the stack that form the second VCSEL comprise a fourth section that forms a mirror that is both sufficiently reflective of the long wavelength light to support lasing at the long wavelength and sufficiently transmissive of the short-wavelength light to provide optical pumping of the second VCSEL. The fifth section forms the cavity that includes the gain medium of the long-wavelength laser and is advantageously wider than it would normally be so that it can include a plurality of quantum wells. The sixth section forms a mirror with the fourth section boundaries of a cavity resonant at the long wavelength and with the mirror of the first section boundaries of a cavity resonant at the short wavelength. Accordingly the sixth section needs to be highly reflective at both the short and long wavelengths. It also needs to be sufficiently transmissive of the long wavelength to provide a useful output.
In the preferred embodiment, the device is formed largely as a stack of the first five sections as layers epitaxially grown on a semi-insulating substrate, such as insulating monocrystalline GaAs. On this substrate is first formed the outer mirror of the short-wavelength VCSEL by a series of quarter short-wavelength layers alternately of the AlAs and AlGaAs to form a distributed Bragg reflector (DBR). Next there is formed the cavity of the short-wavelength laser as a layer of GaAs in which is formed a P-I-N diode. Next there is formed the second or inner mirror of the short-wavelength VCSEL, also by a succession of one-quarter wavelength short-wavelength layers, alternately of AIAs and AIGaAs to form a DBR. This marks the end of the short-wavelength laser. Next there follows a succession of one-quarter long-wavelength layers alternately of AIAs and GaAs to form the inner DBR mirror of the long-wavelength cavity followed by layers that form the long-wavelength AIGaAs cavity, sufficiently wide to include at least two layers of GaAsSb and GaAs that form the quantum wells that serve as the gain medium for the long wavelength VCSEL. Finally, as the outer mirror of the long-wavelength VCSEL there is deposited a plurality of dielectric layers, alternately of SiO2 and TiO2 to form a broad band mirror highly reflective of both the short and long wavelength.
In particular, distance separating the inner mirror of the short-wavelength VCSEL and the outer mirror of the long-wavelength VCSEL is such as to establish a standing wave of the short-wavelength therebetween.
The invention will be better understood from the following more detailed description taken in conjunction with the accompanying drawing.